A study of "worms" melt fracture in polyethylene extrusion blow molding process using capillary rheometry

During the polyethylene (PE) blow molding process of large size drums, string‐like defects, which are referred to here as worm melt fracture, can sometimes be observed on the extrudate surface. Such string‐like defects, in various shapes and sizes, are also observed in the capillary extrusion at high shear rates after the slip‐stick transition. The PE resin with broader molecular weight distribution (MWD) exhibits a greater degree of worm melt fracture while narrow MWD PE resin, which has higher slip velocity and a uniform slip layer, shows a lesser degree of worm melt fracture. It is hypothesized that the worm melt fracture is related to the die build‐up. Based on the mechanism of the fast die build‐up, it is proposed that the cohesive slip layer, which is a failure within the polymer melts at an internal surface, could emerge out from the die as these string‐like materials attached on the extrudates. The broader MWD resin, which has more small polymer chains and a lower plateau modulus, is postulated to have a weaker polymer melt, which then makes it easier to have such an internal failure and consequently have more string‐like defects at high shear rates. POLYM. ENG. SCI., 56:650–656, 2016. © 2016 Society of Plastics Engineers